Hydraulic brake fluid



Patented Aug. 15, 1939 UNITED STATES PATENT OFFICE HYDRAULIC BRAKE FLUIDNo Drawing. Application July 10, 1937, Serial No. 152,989

6 Claims.

This invention relates to operating fluids for hydraulic brake systemsand to fluid pressure operating devices generally. More particularly, itrelates to the combination among themselves of certain alkyl ethers ofthe alkylene and polyalkylene glycols, which mixtures in turn arecombined with suitable nonmineral lubricating oils, such as castor oil.

Hydraulic brake fluids generally include an oil as an essentialcomponent, and vegetable and animal oils of low solidifying point arecommonly used for this purpose. These fluids must be completely misciblewith other oleaginous or fatty acid-base pressure transmitting media, inorder that the brake systems of automobiles may be readily-serviced.Refill solutions require miscibility with solutions already in thesystem in order to prevent separation of the components under conditionsof use. These fluids should not corrode metal surfaces used in thehydraulic brake system. Nor should they cause deterioration or swellingof the rubber pistons or tubes which they contact in use. Therubber-swelling action of a hydraulic brake fluid is a very importantfactor in the efficiency of the fluid, and, in no case should theswelling action be greater than from 5% to 7% by volume of the rubbermember. Again, such fluids should remain liquid over a temperature rangeof from at least 50 to 220 F. This is imperative if the fluids are notto congea-l at low temperatures, thereby preventing operation of thebrake system, or to vaporize at high temperatures, causing a separationof the components and vapor locking, with consequent inoperativeness ofthe system. The solubility characteristics of a suitable fluid for usein pressure transmission devices must be such as to permit toleration ofwater which may be present in, or find its way into, the system. It hasbeen found that desirable fluids should have a water tolerance up to andeven greater than 15% by volume of the fluid. This is essential ifseparation of the components is to be avoided, particularly at lowtemperatures.

While many hydraulic brake fluids containing glycol alkyl ethers andcastor oil meet some of these demands, no fluid at present availablesatisfies all of these requirements. This may poss bly be due to thefailure of previous workers to recognize that the varying properties ofthe individual ethers are controlling factors, and the consequentfailure to utilize the useful and valuable properties of each ether inmixtures of them, and in mixtures of special combinations of theseethers in definite proportions with fatty acidcontaining oils ofsuitable lubricating characteristics.

I have found that when certain glycol monoalkyl ethers, namely thosehaving less than three carbon atoms in their alkyl radical, are combined5 with other monoor dialkyl ethers having at least three carbon atoms inthe alkyl substituents, the resulting mixture exhibits unexpectedproperties. This combination of properties, which is of paramountimportance, includes a low solvent 10 action on rubber (a minimum ofrubber-swelling action), and complete miscibility with castor oil. Thisdesirable combination of properties, when the mixture is combined withcastor oil, is accompanied by low corrosive action on metal sur- 1faces, pronounced chemical stabilty, and by water tolerance well withinthe desired limits.

The advantages of the special combination of these particular ethersarise from two very important factors. First, those alkylene or polyal-20 kylene glycol monoalkyl ethers which have less than three carbonatoms in their alkyl radical, while possessing exceptionally low solventaction on rubber, or rubber-swelling action, are relatively immiscibleat low temperatures with castor 25 oil. As was noted above, this lack ofmiscibility would markedly decrease the efliciency and dependability offluids in which such compounds were used as the sole or major diluent.The second important feature is that those alkylene and 0 polyalkyleneglycol monoor dialkyl ethers having three or more carbon atoms in theiralkyl radicals, while possessing excellent miscibility with castor oil,are characterized by an excessive and damaging swelling action on therubber members 36 of the system. When two ethers of these respectiveclasses are combined, in proportions set forth hereinafter, and thiscombination of ethers is admixed with castor oil, a brake fluid isobtained which more than satisfies the demands imposed 40 on such mediaby conditions of use. For example, with my new fluids, rubber swellingor expansion of rubber members in use, does not exceed 3.5% to 4.0%. Theimproved fluids are without material corrosive effect on aluminum, 45copper or other metal surfaces which they may contact in use, My newfluids are sufliciently liquid for practical use at temperatures of from50 F, to 220 F. Furthermore, they are completely miscible with castoroil or other oleagi- 50 nous base-containing fluids.

Broadly, the glycol ethers for use in this invention consists ofmixtures of the monoand dialkyl ethers of 1.2 glycols of the ethylene,propylene, butylene and amylene series, and their 55 homologues. Forexample, the monomethyl or monoethyl ethers of mono-, di-, tri-, ortetraethylene glycols are admixed with the monoisopropyl. butyl, or amylethers, or mixtures thereoi, or, the monomethyl or monoethyl ethers ofethylene and polyethylene glycols can be admixed with the monobutylether of ethylene or diethylcne glycol or of tripropylene glycol, or ofothers of this series. In all such combinations, the glycol monomethylor monoethyl ethers retain the greater part of their low solvent actionon rubber while the miscibility of the mixture with casior oil isgreatlyincreased by the effect of the glycol ether containing higheralkyl groups and which serves as a coupling agent. The glycol monoordialkyl ethers having three or more carbon atoms in their alkyl groupsretain their excellent miscibility with castor oil, and the markedtendencies of these compounds otherwise to exert high solvent action onrubber are rendered practically negligible by the presence of thecompounds having less than three carbon atoms in their alkyl group.

I have found that very effective fluids are obtained when the alkyleneand polyalkylene glycol monoalkyl ethers containing less than threecarbon atoms in their alkyl radical are used in approximately 30% to 45%by volume, and the alkylene or polyalkylene glycol monoor dialkyl etherscontaining at least three carbon atoms in the alkyl substituents areused in amounts of from about 15% to 20% by volume, together with castoroil in amounts of from 35% to 50% by volume.

A specific example of a fluid according to this invention may contain35% of ethylene glycol monoethyl ether, 15% of diethylene glycolmonobutyl ether, and 50% of castor oil, all by volume. Another fluid wasformed of 40% of diethylene glycol monomethyl ether, 15% of dipropyleneglycol monobutyl ether, and 45% of castor oil, all by volume. A thirdwas composed of 30% of triethylene glycol monoethyl ether, 25% ofpropylene glycol monobutyl ether, and 45% of castor oil, all by volume.These three specific compositions are merely illustrative of the scopeof the invention and other combinations of compounds of the classes setforth have proved to be equally effective.

The mixtures shown above in no instance caused rubber swelling of morethan 5%, nor did they absorb water to the extent of more than to 11%,and in all respects these compositions manifested characteristics ofhighly satisfactory and dependable fluids.

It must be understood that the low rubberswelling action and completemiscibility with castor oil are two controlling features governing thecomposition of these fluids, and, while the percentages shown above havebeen found effective, it is possible that variations in theseproportions may be used without departing from the spirit of thisinvention. For instance, the varying conditions of operation to whichthe fluids will be subjected might determine that more of the one andless of the othershould be employed.

If found desirable in practice, certain viscosity-reducing agents may beadded to the hydraulic brake fluids. I have found that the use of mostviscosity-reducing agents does not impair the efllciency of my newfluids. If such agents are desired, the ketones, such as acetone, methylisobutyl ketone, and the like; lower fatty acid alkyl esters, such asbutyl acetate and isopropyl acetate; and lower monohydric alcohols, suchas ethanol and methanol, are suitable. Of course, it will be appreciatedthat the uses for which the fluids are intended will control the natureand quantity of the viscosity-reducing agents employed, i. e., thehigher the temperature to which the fluid is to be subjected, the lessvolatile should be the viscosity-reducing agent. At any rate, if theseagents are to be employed, it is recommended that they be used inamounts not greater than about 10% by volume. A very effective fluidemploying such viscosity-reducing agents may be formed of 30% ofmonoethyl ether of ethylene glycol, 15% of monobutyl ether ofdipropylene glycol, 45% of castor oil, and 10% of butyl acetate, all byvolume.

The present invention provides compositions of matter having useful andimproved operating characteristics as pressure transmission media.Specific preparations have been set forth, but the proportions shown areillustrative only, and the invention is susceptible of modificationwithin its scope as defined by the appended claims.

I claim:

1. A hydraulic brake fluid comprising a glycol ether from the groupconsisting of alkylene and polyalkyiene glycol monoalkyl ethers havingnot more than two carbon atoms in the alkyl radical in admixture with amaterially less amount of a glycol ether from the group consisting ofthe monoand dialkyl ethers of the monoand poly alkylene glycols havingat least three carbon atoms in the alkyl radical thereof and combinedwith an amount of castor oil at least as great as any other ingredient,said brake fluid being characterized by a greater miscibility at lowtemperature with castor oil than fluids comprising the first group ofthe aforesaid glycol ethers and being further characterized by a lesserswelling action for rubber than fluids comprising the second group ofthe aforesaid glycol ethers.

2. A hydraulic brake fluid comprising a glycol ether from the groupconsisting of alkylene and polyalkylene glycol monoalkyl ethers havingnot more than two carbon atoms in the alkyl radical in admixture with amaterially less amount of a glycol ether from the group consisting ofthe monoand dialkyl ethers of the monoand polyalkylene glycols having atleast three carbon atoms in the alkyl radical thereof and combined withan amount of castor oil greater by 15% by volume than any otheringredient, said brake fluid being characterized by a greatermiscibility at low temperature with castor oil than fluids comprisingthe first group of the aforesaid glycol ethers, and being furthercharacterized by a lesser swelling action for rubber than fluidscomprising the second group of the aforesaid glycol ethers.

3. A hydraulic brake fluid comprising about 30% to about 40% by volumeof a glycol ether from the group consisting of alkylene and polyalkyleneglycol monoalkyl ethers having not more than two carbon atoms in thealkyl radical, about 15% to about by volume of a glycol ether from thegroup consisting of the monoand dialkyl ethers of the monoandpolyalkylene g1ycols having at least three carbon atoms in the alkylradical, and the remainder an amount of castor oil at least equal to theamount of any other component present, said brake fluid beingcharacterized by a greater miscibility at low temperature with castoroil than fluids comprising the first group of the aforesaid glycolethers, and being further characterized by a lesser swelling action forrubber than fluids comprising the second group of the aforesaid glycolethers.

4. A hydraulic brake fluid essentially containing about 35% by volume ofethylene glycol monoethyl ether, 15% by volume of dlethylene glycolmonobutyl ether and 50% by volume of castor oil.

5. A hydraulic brake fluid essentially containing 40% by volume ofdlethylene glycol monomethyl ether, 15% by volume of dlpropylene glycolmonobutyl ether, and 45% by volume of castor oil.

6. A hydraulic brake fluid essentially contain- -ing 30% by volume oftriethylene glycol monoethyl ether, 25% by volume of propylene glycolmonobutyl ether, and 45% by volume of castor oil.

- HARVEY R. FIFE.

